1. Field of the Invention
The present invention relates generally to a method and system for suppressing fluctuation of width in a hot rolled strip or sheet metal, in a hot mill line. More specifically, the invention relates to a technique for cooling hot rolled strip or sheet metal transferred from a finishing mill to a coiler and suppressing fluctuation of width.
2. Description of the Background Art
In general, hot rolled strip is transferred from a finishing mill to a coiler in a hot mill line. When the leading edge of the hot rolled strip reaches the coiler and is coiled by the coiler, an impulsive tension force may be exerted on the strip. This impulsive tension force is transmitted throughout the hot rolled strip between the finishing mill and the coiler. As is well known, such impulsive tension force may particularly apply to the portion of the strip at a position downstream of the finishing mill, up to several tens of meters to serve as a force causing longitudinal expansion. Consequently, necking may occur at the portion where the impulsive tension force is applied to reduce the width of the strip.
Namely, in the conventional hot mill line, the hot rolled strip from the finishing mill is transferred through a run-out table and cooling stage where a cooling device discharging cooling water toward the hot rolled strip is provided, to the coiler. A pair of pinch rollers are provided in the vicinity of the coiler for assisting coiling. In the usual layout of the hot mill line, the finishing mill and the coiler are distanced at about 150 meters. Along the path of the hot rolled strip between the finishing mill and the coiler, a thickness gauge, a shape monitor, a width gauge, a thermometer and so forth are arranged. These strip condition monitoring facilities are generally provided in the vicinity of the outlet of the finishing mill. In order to allow arrangement of these strip condition monitoring facilities, a distance of about 10 meters has to be provided between the finishing mill to the inlet of the cooling stage. Therefore, the hot rolled strip from the finishing mill has to be transferred in an uncooled condition for about 10 meters.
On the other hand, in order to hold the coiling performance and configuration of the end of the coil in good condition, the coiler should be driven at a leading speed which is 1.1 to 1.3 times higher than the line speed of the strip. Due to this difference of the speed between the coiler and the strip, an impulsive tension force may be generated at the beginning of coiling. This impulsive tension force causes local necking particularly at portions of the strip where deformation resistance is small. In our experience, it has been appreciated that the impulsive tension force particularly locally affects the configuration of the strip at the portion about 20 meters from the finishing mill to cause local necking.
Once the leading end of the strip is coiled by the coiler, the coiler speed becomes synchronous with the line speed of the strip. At the portion of the strip following the portion where necking is observed, hunting in width to fluctuate the width of the strip occurs. Such hunting in width is considered to be caused by temperature differences influenced by skid marks at the outlet of the finishing mill and/or by the relationship between the hot strength of the strip and the unit tension.
In order to suppress necking and hunting as set forth above, the Japanese Patent First (unexamined) Publication (Tokkai) Showa 59-10418 discloses a system including a looper or pinch rollers vertically movable between the finishing mill and the coiler. The looper and pinch roller are responsive to the tension force to be exerted on the hot rolled strip for providing an extra length of strip in order to absorb the extra tension force and regulate the tension force to be exerted on the strip.
On the other hand, the Japanese Patent First Publication (Tokkai) Showa 56-56705 discloses a method for absorbing the impulsive tension force by means of pinch rollers. In the disclosure, the pinch rollers pinch the hot rolled strip, hold the strip until the coiler speed becomes synchronous with the line speed, and release the pinching force after the tension is substantially regulated.
Furthermore, the Japanese Patent First Publication (Tokkai) Showa 49-23751 proposes to provide a greater width for the portion of the hot rolled strip where necking possibly occurs. The extra width to be provided for the possible portion to cause necking, will be determined at a value corresponding to reduction of the magnitude of the width due to necking. In the alternative, the Japanese Patent First Publication Showa 49-23751 also proposes a technique able to perform rapid cooling for the strip so as to provide sufficient deformation resistance to the strip for preventing the strip from causing deformation including necking.
In the Japanese Patent First Publication 59-10418, since the extra length of the strip is provided through the looper or pinch rollers, tension force at the initial stage becomes insufficient to hold the coiled leading end portion of the strip in good configuration. Especially, when waving is caused in the strip, the length of the strip to be provided by the looper or pinch roller becomes too great to make it possible to establish the metal strip coil in the desired coil configuration. On the other hand, in case of the Japanese Patent First Publication 56-56705, the pinch rollers should be provided a pinching force substantially corresponding to the possible impulsive tension force. Therefore, relatively bulky construction of the pinch roller is required, increasing the facility cost. Furthermore, in order to drive such bulky construction of the pinch rollers, relatively large electric power would be consumed. In addition, in case of thin strip which tends to cause waving an extra length of the strip may be provided between the pinch rollers and the coiler to reduce the tension force to be exerted on the strip therebetween. In the worst case, the substantial waving of the strip may provide an extra length of the strip to lose tension to be exerted on the strip. Therefore, similarly to the technique of the Japanese Patent First Publication 59-10418, the tension force becomes insufficient to hold a good coil configuration. In such case, in order to make the coil configuration in good shape, the mandrel of the coiler has to be accelerated again after the pinch rollers are released. By accelerating the mandrel, impulsive tension force may be exerted on the strip to cause necking and/or hunting.
In case of the Japanese Patent First Publication 49-23751, in order to satisfactorily and completely compensate the reduction of the strip width in necking, it is necessary to provide the extra width in the portion of 50 meters in length which corresponds to 7 to 8 meters of the sheet bar. On the other hand, the longitudinal region to cause necking is about 20 meters. Therefore, the extra width of the strip may be lend in a as length of 30 meters. When the coil with the extra width portion is processed in the cold mill line for example, edge folding may occur at the portion where the extra width is maintained when an edge portion control is performed. In order to avoid the possibility of edge folding, a slow-down line speed in the cold mill line becomes necessary.
Consequently, the conventionally proposed systems are not satisfactory in suppressing necking and/or hunting of the strip width, at all.